dlj04(at)josephson.com
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 Posted: Sat Jun 21, 2014 11:40 am Post subject: fuse link to replace fuse in low current applications? |
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>The concept of a "fuse" to keep faults in a tachometer from
>disabling an ignition needs to be stamped out, it is a dangerous
>misconception. I know of at least one incident where this almost
>caused a fatal crash. It takes very little current through a P lead
>from a magneto or points wire in a Kettering ignition to disrupt
>performance of the ignition. Can you depend on this amount of
>current blowing a fuse? Is the resistor going to open if there is a
>fault? Is the engine going to keep running well if the far end of
>the resistor is shorted to ground or +12? Not unless it has been
>tested to do so.
Yes . . . these techniques have been tested many times . . .
except for the fuse . . . I don't recall seeing fuses
recommended for this application but agreed . . . they're
a bad idea.
Having said that, let us consider the use of resistors to
sample the really trashy signal that appears across the points
of either a magneto or Kettering system.
<snip>
Yes, a resistor would be the right answer, and easy enough to test for
whether ignition operation is compromised.
>tach could also yield failsafe operation but that's still putting a
>lot of faith in something you haven't tested. The "proof by
>assertion" that I've seen from a few instrument vendors isn't really
>confidence-inspiring.
I've never heard of this being an issue. I've sampled
p-lead and Kettering points through resistive attenuators
dozens of times over the years with nary a concern
for any ill effects being reflected back to the source.
The case I'm aware of involves a glass-cockpit display and a
points-and-coil ignition, which when connected to the ignition lead
caused missing, and would sometimes cut out entirely at higher rpm. I do
not know what the failure mode was, but the makers of this device didn't
seem to understand that the spikes on the line could easily exceed 200
volts. It was rumored that the bypass capacitors on the input lines were
rated at 50 volts, although I have not confirmed that (which is also why
I am not identifying the device.) You can see what would happen if the
spikes caused arcing in a bypass cap, which then failed. The point is,
an external resistor would both provide the attenuation needed and
prevent a failure. You could test it by grounding the instrument end, or
connecting it to +12, and confirming the engine would still run properly
no matter what happened.
Magneto p-lead signals are really trashy, I have
found it useful to run the attenuated signal
through 4 poles of band pass filtering before squaring
it up to drive the digital interpreters.
A fuse doesn't even make sense. The signal being
observed is WAAYyyyy to big as a digital signal
source . . . attenuation on the order of 30:1
is the first thing you need to do with it.
Yes, that's what I was responding to. Much earlier in this thread,
someone suggested that a fuse would prevent an instrument failure from
compromising the ignition. It won't.
David Josephson
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